Conventionally, a liquid crystal display panel has been used in which a first substrate and a second substrate are opposed to each other with a predetermined gap provided therebetween, a liquid crystal layer is sealed in the gap with a sealant, pixel portions are formed by electrodes, the electrodes being provided on the substrates to oppose to each other via the liquid crystal layer, and lead electrodes for applying electric signals to the electrodes forming the pixel portions are provided at least on the first substrate. In such a liquid crystal display panel, the electric signals applied to the electrodes forming the pixel portions change optical characteristics of the liquid crystal layer to control ON/OFF states of the pixel portions for performance of display.
The configuration of such a conventional liquid crystal display panel will be described using FIG. 41 to FIG. 43 taking, as an example, a reflective liquid crystal display panel for use in a cellular phone, a personal digital assistant, a timepiece, and the like. FIG. 41 is a plan view of the conventional liquid crystal display panel, FIG. 42 is a cross-sectional view taken along a line 42—42 shown in FIG. 41, and FIG. 43 is a partially enlarged plan view of a part (in circle C) in FIG. 41.
This liquid crystal display panel is, as shown in FIG. 41, a matrix-type liquid crystal display panel having m stripe first electrodes 2 provided on a first substrate 1 and n stripe second electrodes 7 provided on a second substrate 6, and a display region 23 constituted of m by n pixel portions 24 being intersections of the first electrodes 2 and the second electrodes 7. The first substrate 1 and the second substrate 6 are opposed to each other with a predetermined gap provided therebetween with not-shown spacers and are bonded together with a sealant 26 as shown in FIG. 42, and a liquid crystal layer 25 is sealed in the gap and hermetically sealed with a closing member 27 so that hermeticity is ensured.
Further, as shown in FIG. 42, a reflector 16 composed of an aluminum film or a silver alloy film is provided on the second substrate 6, and a color filter which is composed of a red (R) color filter 17, a green (G) color filter 18, and a blue (B) color filter 19 is provided on the reflector 16. Thereon, a flattening protective film 21 is provided to flatten projections and depressions of the color filter and prevent an electrical short circuit between the reflector 16 and the second electrodes 7, and the second electrodes 7 are provided on the flattening protective film 21. Furthermore, on the first electrodes 2 and on the second electrodes 7, alignment films (not shown) are provided to align liquid crystal molecules in the liquid crystal layer 25 in predetermined directions.
By the way, in this liquid crystal display panel, as shown in FIG. 41, the first substrate 1 is made larger in size than the second substrate, and a driving integrated circuit (IC) 36 for applying driving signals to the first electrodes 2 and driving ICs 35 for applying driving signals to the second electrodes 7 are mounted on the first substrate 1. Note that the second substrate 6 is made to have a size larger than the display region 23 and not to reach a region where the driving ICs 35 and 36 are provided on the first substrate 1.
Then, lead electrodes continued to the first electrodes 2 for connecting the first electrodes 2 and the driving IC 36 are led out from the display region 23 to the outside of the sealant 26. On the lead electrodes, the driving IC 36 is mounted through an anisotropic conductive film containing conductive particles in a polyimide resin, and the film is compressed by heat to cure, so that the first electrodes 2 are connected to the driving IC 36 through the lead electrodes.
Further, lead electrodes 41 for connecting the second electrodes 7 and the driving ICs 35 are provided on the first substrate. A portion of the sealant 26 is composed of an anisotropic conductive sealant containing conductive particles in an acrylic resin, and a pressure is applied to the second substrate 6 and the first substrate 1 through the anisotropic conductive sealant, so that the second electrodes 7 provided on the second substrate 6 are electrically conducted through the conductive particles to the lead electrodes 41 provided on the first substrate 1. Then, the driving ICs 35 are mounted on the lead electrodes 41 similarly to the case of the above-described driving IC 36, so that the second electrodes 7 are connected to the driving ICs 35 through the lead electrodes 41.
Further, to apply signals to the driving ICs 35 and 36 from an external circuit, a flexible printed circuit board (FPC) 31 connected to the driving ICs 35 and 36 through connecting electrodes 42 shown in FIG. 42 and FIG. 43 is provided. Note that the FPC 31 and the driving ICs 35 and 36 are connected to the connecting electrodes 42 through an anisotropic conductive film.
In such a conventional liquid crystal display panel, as shown in FIG. 43, an insulating resin 32 is applied to a portion where the lead electrodes 41 are provided on the first substrate 1 to prevent occurrence of a potential difference between adjacent lead electrodes 41 and adherence of contamination or moisture thereto. As this insulating resin 32, an epoxy resin having low moisture permeability or a silicon resin having low moisture absorbability is used.
However, in a fabrication process of a liquid crystal display panel body, a mounting process of the driving integrated circuits 35, or a mounting process of the FPC 31, if contamination adheres thereto, pinholes form in the insulating resin 32, or the insulating resin 32 has an insufficient moisture blocking property, the electrode material melts (is corroded) at a portion of the lead electrode 41 to create an electrolytically corroded portion 47 as shown in FIG. 43, finally giving rise to a phenomenon that the lead electrode 41 is broken, when the liquid crystal display panel is driven for a long time or in an atmosphere at a high temperature and high humidity. Therefore, when the electrolytically corroded portion 47 is created, the driving signals from the driving ICs 35 and 36 cannot be transmitted to electrodes constituting the pixel portion 24, which makes it impossible to perform intended display, and as a result, the display quality of the liquid crystal display panel is significantly reduced.
The connecting electrode 42 for establishing connection with the FPC 31 can have an electrode width and gap between electrodes which are larger than those of the lead electrode 41 and thus can be structured to be relatively insusceptible to electrolytic corrosion. On the other hand, the lead electrode 41 has to have a small electrode width and gap between electrodes to increase the pixel density within the display region 23 and thus becomes susceptible to electrolytic corrosion.
Therefore, it is very important to prevent the electrolytically corroded portion 47 from being created in the lead electrode 41 even when the liquid crystal display panel is used in the atmosphere at a high temperature and high humidity, in order to increase the usable range of the liquid crystal display panel and keep good display quality for a long time. Besides, since there is a strong demand for a reduction in size and cost of the liquid crystal display panel, it is also important to prevent the electrolytically corroded portion 47 from being created without greatly departing from the thickness and size of the conventional liquid crystal display panel and with suppressing an increase in cost and weight to a minimum. More than that, an electrolytic corrosion preventing structure is necessary which can cope with various kinds of methods for connecting lead electrodes to external circuits and driving ICs.
It should be noted that a structure shown in FIG. 44 is also well known as a connecting structure for preventing moisture entrance into a connecting portion in a flat display panel in which an FPC is directly connected to a lead electrode.
This flat display panel is a thin film EL (electro luminescent) display panel in which a lead electrode 94 provided on a glass substrate 91 is bonded and connected to an FPC 95 with a solder 96, a resin 97 is filled in a region including the connecting portion on the glass substrate 91, and further a protective glass plate 98 is disposed on the resin 97.
The protective glass 98 provided on the resin 97 as described above can decrease the area of the resin 97 in contact with air and prevent moisture entrance into the lead electrode 94 and corrosion of the lead electrode 94.
Even in this structure, however, it is conceivable that moisture enters through between a rear glass substrate 92 and the protective glass plate 98, and thus it cannot be said that the structure is sufficient in moisture blocking property and accordingly electrolytic corrosion preventing ability.
It is an object of the present invention to solve such problems and greatly reduce occurrence of electrolytic corrosion at a lead electrode using a simple technique with the size and weight of a conventional liquid crystal display panel being secured.